AR-15 Over-The-Beach Yes or No?

A SEAL could probably answer that question.

The trick is to open the bolt slightly and dump the water out of the barrel.
A larger bore firearm will drain on it's own, but the small bore of the AR series may retain the water

Navy SEALs and some other Special Ops units that work in the water regularly go swimming with their M4's and they work fine.
After use in salt water they flush them with fresh water.
I suspect that if they're going to be in the water very long they may use a heavier lubricant that won't wash off easily.

If you do some searches you can find any number of photos of people coming out of the water with M4's

Condoms are a SEAL's best friend, think they still use them to cover the muzzle to keep water out. Condoms work great, keeps water out and don't have to remove before firing. If not using anything to cover the muzzle, when coming out of the water, just pull back on the charging handle a bit to drain the water. Biggest thing to worry about, is mud in the barrel, not as easy to notice.

Plastic muzzle covers were issued for M16s back in the 60s. They fit tightly over the birdcage flash supressor and did a decent job of keeping crap out of the bore. I don't know how well they'd do with prolonged submersion of the rifle. They worked well with a round in the chamber for short durations of immersion. If you had to shoot you didn't have to remove the muzzle cover. It would just shoot off.

A field expedient used was a piece of plastic and rubber band or tape.

Condoms were used over muzzles in WW2. The brass didn't want photos showing soldiers with this. A canvas muzzle cover was made that fit the Garand, M1 carbine, and 03 Springfield.